CLASS XII CHEMISTRY

DISADVANTAGES OF MENDELEEV'S PERIODIC LAW 

In spite the above advantages Mendeleev's periodic table suffered defects as follows :

1. The position of Hydrogen was not correctly defined.it was placed in group one I although it resembles with both the group I elements - the Alkali metals and the group VII elements - the Halogens , in their properties. 

2. In some cases Mendeleev placed elements according to the similarities in the properties and not in the increasing order of their atomic masses.Thus position of these elements was not justified. e.g cobalt (atomic mass 58.9) was placed before Nickle (atomic mass 58.6).

3. Isotopes were not given in separate places in the periodic table although Mendeleev's classification is based on the atomic masses.

4. Some similar elements were grouped separately while some dissimilar elements were grouped together.For example copper and mercury are similar in their properties but were placed separately . Copper was placed in group I although it did not resemble the elements of this group.

5. Mendeleev could not explain the cause of periodicity in the elements.

6. The position of lanthanides and actinides were not included in this periodic table.